We are fairly well equipped for our own cruising but don't have enough life jackets to take many passengers out with us yet. How good are buoyancy aids? We wouldn't be taking passengers with us in extreme conditions (intentionally) obviously good life jackets aren't cheap but I wouldn't want to compromise anyone's survival because of cost.

I completely see where you are coming from. You could get an acceptable lifejacket for £29 or less. For guests it would surely need an auto all the way and that seems to be about £45, so 2 x LJs cost the same as 3 BAs. Added to that if your guest does go for a swim they cost you money... BUT - BAs are more size specific so if you have a lardy friend and a salad muncher they ain't gonna fit in the same size BA and expect it to work properly.

So while I would instinctively say LJ all the way I dont think its a daft question. But probably needs more factors:
Crusing water - if you did something daft and all occupants end up swimming time to be located and rescued? Thats probably a factor of loads of things - actual location, other vessels, safety kit like PLBs carried etc.
If you were inland in say Loch Lommond then while swimming to shore is rarely the 'right' thing to do its possible. If you are crossing a bay and will be 10 miles off any point of land its not an option...
Activities - how likely will you guest want to get in the water and cost you money!
BAs offer a good bit of insulating protection - what other gear will you carry for them (waterproofs etc?)
I'd then say you probably want to look at the guidance on spray hoods, crotch straps and lifejacket lights. But then if you think any of that is a good idea for the life jacket you've just answered the question as there is no way a BA is going to be the right thing.

I carry 2 x manual LJs for occasional passengers / unforeseen circumstances, plus a rearming kit for our autos. These are the original LJs we bought, and since uprated to autos now we have a bigger / more powerful boat and venture further / faster.

What who carry depends on what you feel comfortable providing, and the trips you will do with non-regular passengers. Most other people we take out have their own LJs and prefer them to a "spare".

If I was taking some friends / friend's children up & down the river at 6kts I'd be happy for them to have BAs, if I was taking them out to sea for a 30kt blast I'd insist on properly fitting LJs.

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Good question & one I'm not sure there's a correct answer to. Up to around 10 years old, our daughter had (good) BA jackets, Crewsavers iirc. They offered good insulation & didn't rely on any gizmo to work. If she went in at least I knew she'd float. We all now have autos, I carry a couple of spares & a re-arming kit. There's been a couple of occasions when I've thought that manual jackets would have been more useful, especially launching & recovering etc. I think for small kids, BA is the way to go, for adults, manual or auto LJ, there's a case for both. I'm currently looking at Hybrids for my next pair, as all mine are coming up for service. So I might treat us to a couple of these next year & opt for the manual version.
We never differentiate what kind of trip we are making, if we are out on the boat, we are out on the boat..

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